What is Class C soil type?

Answer 1

Site Class C is a soil type defined in the 2009 International Building Code as a "very dense soil and soft rock", Chapter 16, Table 1613.5.2.

Soil site classes are determined either by soil type, (eg. soft clays, sands and gravels, weathered-soft rock, hard rock etc.) or by shear wave velocity testing.

The denser the soil, the faster shear waves and earthquake forces will travel thorough the ground. Slower seismic waves in softer soils create higher deformations and exert greater forces on buildings.

The U.S. Geological Survey produces National Seismic Hazard Maps which determine spectral responses, another major factor in the seismic design provisions of building codes.

Our understanding of seismic forces based on soil type and spectral response coupled with knowledge of building materials allows us to design better buildings, bridges, highways, and critical infrastructure which can better withstand earthquakes, saving lives and reducing disruption to critical activities following a seismic event.